Visna Virus is the cause of a demyelinative disease of sheep with long incubation period and protracted clinical course characteristic of slow virus disease. We have demonstrated that this virus biochemically is related to RNA tumor viruses and replicates via a DNA intermediate or provirus. This proposal addresses the implications of this finding for slow viruses and persistence of virues in the central nervous system. We will investigate synthesis and physical nature of the DNA provirus and transcription of viral specific RNA. The salient findings from the lytic cycle will form the basis of comparable investigations of molcular events in cultured nervous tissue and in annmals infected with visna virus.